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THE SENSE OF SMELL. The Sense of Smell is located in the mucous membrane lining the upper part of the nasal cavity, in which the olfactory nerves are distributed.

The Nasal Fossæ are two cavities, irregular in shape, separated by the vomer, the perpendicular plate of the ethmoid bone, and the triangular cartilage. They open anteriorly and posteriorly by the anterior and posterior nares, the latter communicating with the pharynx. They are lined by mucous membrane, of which the only portion capable of receiving odorous impressions is the part lining the upper one-third of the fossæ.

The Olfactory Nerves, arising by three roots from the posterior and inferior surface of the anterior lobes, pass forward to the cribriform plate of the ethmoid bone, where they each expand into an oblong body, the olfactory bulb. From its under surface from fifteen to twenty filaments pass downward through the foramina, to be distributed to the olfactory mucous membrane, where they terminate in long, delicate, spindle-shaped cells, the oifactory cells, situated between the ordinary epithelial cells.

The olfactory bulbs are the centers in which odorous impressions are perceived as sensations, destruction of these bulbs being attended by an abolition of the sense of smell.

In animals which possess an acute sense of smell there is a corresponding increase in the development of the olfactory bulbs.

The Essential Conditions for the sense of smell are (1) a special nerve center capable of receiving impressions and transforming them into odorous sensations. (2) Emanations from bodies which are in a gaseous or vaporous condition. (3) The odorous emanations must be drawn freely through the nasal fossæ; if the odor be very faint, a peculiar inspiratory movement is made, by which the air is forcibly brought into contact with the olfactory filaments. The secretions of the nasal fossæ probably dissolve the odorous particles.

Various substances, as ammonia, horseradish, etc.., excite the sensibility of the mucous membrane, which must be distinguished from the perception of true odors.

THE SENSE OF SIGHT. The Eyeball.— The eyeball, or organ of vision, is situated at the fore part of the orbital cavity and supported by a cushion of fat; it is protected from injury by the bony walls of the cavity, the lids and lashes, and is so situated as to permit of an extensive range of vision. The eyeball is loosely held in position by a fibrous membrane, the capsule of Tenon, which is attached on the one hand to the eyeball itself and on the other to the walls of the cavity. Thus suspended, the eyeball is capable of being moved in any direction by the contraction of the muscles attached to it.

Structure.—The eyeball is spheroidal in shape and measures about ninetenths of an inch in its antero-posterior diameter, and a little less in its transverse diameter. When viewed in profile it seen to consist of the segments of two spheres, of which the posterior is the larger, occupying five-sixths, and the anterior the smaller, occupying one-sixth of the ball.

The eye is made up of several membranes concentrically arranged, within which are enclosed the refracting media essential to vision. These membranes, enumerated from without inward, are: Ist, the sclerotic and cornea; 2d, the choroid and iris; 3d, the retina ; the refracting media are the aqueous humor, the crystalline lens, and vitreous humor.

The Sclerotic and Cornea—The sclerotic is the opaque fibrous membrane covering the posterior five-sixths of the ball. It is composed of connective tissue arranged in layers which run both transversely and longitudi. nally; it is pierced posteriorly by the optic nerve about one-tenth of an inch internal to the optic axis. The sclerotic by its density gives form to the eye and protects the delicate structures within it, and serves for the attachment of the muscles by which the ball is moved.

The cornea is a transparent non-vascular membrane covering the anterior one-sixth of the eyeball. It is nearly circular in shape and is continuous at the circumference with the sclerotic, from which it cannot be separated. The substance of the cornea is made up of thin layers of delicate, transparent fibrils of connective tissue more or less united together; between these layers are found a number of inter-communicating lymph spaces lined by endothelium, which are in connection with lymphatics. Leucocytes or lymph corpuscles are often found in these spaces. The anterior surface of the cornea is covered by several layers of nucleated epithelium which rest upon a structureless membrane known as the anterior elastic lamina. The posterior surface is covered by a similar membrane, the membrane of Descemet, which becomes continuous at its periphery with the iris; it is also covered by a layer of epithelial cells. At the junction of the cornea and sclerotic is found a circular groove, the canal of Schlemm.

The Choroid, the Iris, the Ciliary Muscle, and Ciliary processes together constitute the second or middle coat of the eyeball.

The choroid is a dark-brown membrane which extends forward nearly

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to the cornea, where it terminates in a series of folds, the ciliary processes. Inits structure the choroid is highly vascular, consisting of both arteries and veins. Externally it is connected with the sclerotic by connective tissue ; internally it is lined by a layer of hexagonal pigment cells which, though usually classed as belonging to the choroid, is now known to belong, embryologically and physiologically, to the retina. From without inward may be distinguished the following layers :

1. The lamina supra choroidea. 2. The elastic layer of Sattler, consisting of two endothelial layers. 3. The chorio-capillaris, choroid proper, or membrane of Ruysch, a

thick, elastic network of arterioles and capillaries lying within the

outer layer of veins and arteries called the vena vorticosæ. 4. The lamina vitrea or internal limiting membrane.

The choroid with its contained blood vessels bears an important relation to the nutrition of the eye; it provides for the blood supply, for drainage from the body of the eye, and presents an uniform and high temperature to the retina.

The iris is the circular variously.colored membrane placed in the anterior portion of the eye just behind the cornea. It is perforated a little to the nasal side of the center by a circular opening, the pupil. The outer or circumferential border is connected with the cornea, ciliary muscle, and ciliary processes ; the free inner edge forms the boundary of the pupil, the size of which is constantly changing. The framework of the iris is composed of connective tissue blood-vessels, muscular fibers, and pigmented connective tissue corpuscles. The anterior surface is covered with a layer of epithelial cells continuous with those covering the posterior surface of the cornea ; the posterior surface is lined by a limiting membrane bearing pigment epithelial cells continuous with those of the choroid. The various colors which the iris assumes in different individuals depend upon the quantity and disposition of the pigmentary granules.

The muscular fibers of the iris, which are of the non-striated variety, are arranged in two sets,— the sphincter and dilator.

The sphincter pupillæ is a circular flat band of muscular fibers surrounding the pupil close to its posterior surface; by its contraction and relaxation, the pupil is diminished or increased in size. The dilator pupilla consists of a thin layer of fibers arranged in a radiate manner; at the margin of the pupil they blend with those of the sphincter muscle, while at the outer border they arch to form a circular muscular layer.

The ciliary muscle is a gray circular band consisting of unstriped muscular fibers about one-tenth of an inch long running from before backward.

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